A reactor core monitoring system in the nuclear power plants is provided with various functions for calculating reactor core performance data, such as power distribution, so as to monitor the soundness of the reactor core. Among the reactor core performance data calculated in a 1-hour cycle or on demand by an operator, thermal limits, power distribution and burnup are targets of monitoring in one example.
The thermal limit herein refers to an instantaneous value which indicates that fuel in the reactor core has no excessive power increase and that cooling by cooling water is effective. The burnup refers to an integrated value obtained by multiplying thermal power of the reactor core by time. The burnup indicates how much nuclear fuel is consumed.
For laborsaving in the plant operation, various monitoring activities are performed to automate control instruments such as control rods. For example, there is known a thermal limit monitoring device for ABWRs, which calculates thermal limits based on process amounts in a short cycle of 200 msec so as to control automatic operation of the control rods (see Japanese Patent laid-Open No. 06-148376).
The calculation results from the thermal limit monitoring device are known to be too conservative. Accordingly, in order to prevent unnecessary interception of control rod operation, the calculation results are corrected by periodically performing calculation with use of reactor core performance data from the reactor core monitoring system as initial values so that the change of state of the plant can be recognized with precision. For this correction, the correction cycle needs to be 5 minutes or shorter. It is required, therefore, to shorten the cycle of calculating the reactor core performance data, which was conventionally 1 hour.
However, if only the calculation cycle is shortened without changing a calculation algorithm for the reactor core performance data in the conventional reactor core monitoring system, an increment of the burnup that is an integrated value of reactor core thermal power, which is calculated based on an integrated value between a present calculation value and a last calculation value, decreases. In this regard, when the increment of the burnup is distributed as three-dimensional data having about 20,000 data points for another calculation such as an isotope weight calculation, the increment value becomes smaller and thereby cancellation of significant digits may occur. As a result, accuracy in calculation of the isotope weight, which is calculated by distributing the burnup as three-dimensional data, may be deteriorated.